Nobody in the fifth or sixth century wrote down anything resembling the King Arthur stories we know today. The earliest possible references appear generations after he supposedly lived, in fragmentary Welsh poems and a ninth century chronicle called the Historia Brittonum, which lists twelve battles a war leader named Arthur supposedly won against invading Saxons. That chronicle was written more than 300 years after the events it describes, by a monk more interested in glorifying British resistance than recording precise history.
Geoffrey of Monmouth turned that thin thread into a legend in 1136 with his History of the Kings of Britain, inventing or embellishing most of what people now consider Arthurian tradition: Camelot, Merlin, Excalibur, and the Round Table. Geoffrey openly borrowed from Welsh folklore, Roman history, and his own imagination, and even contemporaries accused him of fabrication. Later French and English writers added Lancelot, the Holy Grail, and the doomed romance with Guinevere, layering centuries of invented material onto a figure whose historical core, if it exists, has almost vanished underneath.
Some historians argue a real war leader inspired the legend, likely a Romano-British commander organizing resistance against Anglo-Saxon settlers after Rome withdrew from Britain around 410. Candidates proposed over the years include Ambrosius Aurelianus, a documented Romano-British leader mentioned by the sixth century monk Gildas, and various regional chieftains whose names appear in scattered Welsh genealogies. Archaeological sites like Tintagel and South Cadbury Castle show genuine high status occupation in the right period, but nothing found at either site names an Arthur or proves a connection to him.
Other scholars think Arthur began as a purely legendary or mythological figure, possibly derived from Welsh or Breton folklore about a heroic protector, and only got attached to real history later as Britain searched for a unifying national story. Gildas, writing closest to the supposed Arthurian period, describes the resistance against the Saxons in detail and never mentions Arthur by name at all, which many historians consider a significant silence rather than an oversight.
What most specialists agree on is narrower than either camp would like: some form of organized British resistance to Saxon expansion did happen in the fifth and sixth centuries, and it is plausible a real leader, known or unknown to us, took part in it. Whether that leader was called Arthur, or whether Arthur is a name later storytellers attached to a composite of several such leaders, remains unresolved.
Do you think a real war leader inspired the Arthurian legend, or was Arthur invented from folklore alone?
In my opinion, the legend of King Arthur is the amalgamation of the narratives of several prominent leaders from the late Roman withdrawal period through the 7th and 8th centuries. These stories gradually evolved into the well-known King Arthur legend that we recognize today.